1. Field of the Invention
This invention relates to the attenuation of sound propagating in stacks and other gas ducts of relatively large diameter.
2. Review of the Art
In industrial installations, noise problems can arise as a result of the propagation of audible sound through stacks or ducts of relatively large diameter, usually in the direction of gas flow through the duct or stack. Depending upon the origin of the sound, this can have a widely varying spectral content, although for any particular installation, this spectral content will usually remain relatively constant and may have much of its energy concentrated at specific frequencies. Whilst frequencies in the entire audible frequency range from 20 Hz to 20 kHz can represent a source of noise nuisance, the frequency range from 50 Hz to 4,000 Hz, which corresponds to the range of greatest sensitivity of the human ear, causes the majority of noise concerns.
In a stack or duct, sound frequencies above a "cut-off frequency" can propagate not only in a plane mode longitudinally of the stack or duct, but also in higher order modes characterized by repeated reflection of the sound from the walls of the duct. The number of higher order modes depends on the shape of the duct and the extent to which the frequency propagated exceeds the cut-off frequency. Such higher order propagation tends to result in greater off-axis radiation of the sound at an exit from the stack or duct, thus increasing the noise nuisance. The cut-off frequency in a cylindrical duct or stack is, in Hertz, 0.586 c/d, where c is the speed of sound in the duct or stack in meters per second and d is the diameter of the duct or stack in meters. The formula is somewhat different for ducts or stacks of non-circular cross section, but ducts or stacks having the same major cross sectional dimension will exhibit generally similar cut-off frequencies.
Accordingly, ducts or stacks with a cross sectional dimension greater than about 5 centimeters will, at room temperature in air, permit higher order propagation of sound at frequencies lower than 4,000 Hertz, although such propagation is unlikely to be significant in ducts or stacks with a cross sectional dimension less than 10 centimeters, with the problem becoming steadily worse as the duct size increases.
It has been proposed, for example in U.S. Pat. Nos. 4,361,206 (Tsai), 4,368,799 (Wagner) and 4,690,245 (Gregorich et al) to utilize venturis in mufflers for internal combustion engines used in trucks and alike, but typically the exhaust pipes utilized for such engines are of too small diameter for higher mode propagation of sound to present a significant problem, and the venturis are utilized in conjunction with other features to attenuate plane mode propagation of sound through a muffler structure. It is also known, as disclosed in U.S. Pat. No. 3,511,336 (Rink et al) to utilize a venturi provided with sound absorbing walls in a similar application. U.S. Pat. No. 1,964,845 (Dietze et al) discloses a silencing system for a ventilation duct which includes a multi-chamber acoustic filter, the chambers beings connected by tapering sections in order to obtain the smooth and rapid air flow between the chambers, although the sections lack the gradually tapering exit sections typical of conventional venturis.
It is also known to install venturis in ducts or stacks, typically as part of flow measurement or other instrumentation.